Update README.md

Writing about an issue I had not only with this gem but also with pdfkit, It seems to be an issue from wkhtmltopdf.

You can reproduce it by yourselves.

```ruby
pdf = WickedPdf.new.pdf_from_string('<h1>Hello There!</h1><img src="../img/spsp.gif"/><img src="http://rubyonrails.org/images/pages/overview/screencasts2.png"/><img src="..dwadwadaw/dwad.png"/>')

save_path = Rails.root.join('tmp','filename.pdf')
File.open(save_path, 'wb') do |file|
  file << pdf
end
```